The present invention generally relates to the production of a pelletized chemical product, and more particularly to the production of durable ferrous sulfate monohydrate pellets having a desired size, shape, and composition.
There are four principal forms of ferrous sulfate, all of which have different waters of crystallization associated therewith. These forms are as follows:
1) FeSO.sub.4 .multidot.7H.sub.2 O PA1 2) FeSO.sub.4 .multidot.4H.sub.2 O PA1 3) FeSO.sub.4 .multidot.H.sub.2 O PA1 4) FeSO.sub.4 (anhydrous)
FeSO.sub.4 .multidot.7H.sub.2 O is unstable and converts spontaneously under atmospheric temperature, pressure, and humidity conditions to FeSO.sub.4 .multidot.4H.sub.2 O. FeSO.sub.4 .multidot.4H.sub.2 O converts to FeSO.sub.4 .multidot.H.sub.2 O upon the application of heat (e.g. at temperatures above 64 degrees C). FeSO.sub.4 .multidot.H.sub.2 O is a stable product, and will not hydrate in the atmosphere. Finally, FeSO.sub.4 (anhydrous) is a manufactured product, and does not exist in nature unlike the other compositions listed above. All forms of ferrous sulfate are highly soluble in water, generating an acidic pH of about 3-4.
There are two basic sources of ferrous sulfate. First, a substantial amount of ferrous sulfate exists naturally, primarily in regions of the world which are volcanic or have large deposits of sulfur-containing minerals. Natural ferrous sulfate (e.g. FeSO.sub.4 .multidot.7H.sub.2 O) does not exist in large quantities, and is not used substantially for commercial purposes. The second major source of ferrous sulfate involves the production thereof as a by-product of heavy industry. Hundreds of thousands of tons of ferrous sulfate (primarily FeSO.sub.4 .multidot.7H.sub.2 O) are generated in the world annually. In 1990, the United States generated approximately 100,000 tons of ferrous sulfate as chemical by-products of industry, primarily in accordance with the following reaction: EQU 2Fe+2H.sub.2 SO.sub.4 +14H.sub.2 O.fwdarw.2FeSO.sub.4 .multidot.7H.sub.2 O+2H.sub.2
The foregoing reaction typically occurs in the steel industry during the cleaning of steel items with sulfuric acid (H.sub.2 SO.sub.4). This process (commonly known as "pickling") is used to clean steel products including wire, nails, fencing, and the like prior to galvanizing or coating with protective oils/plastics. The FeSO.sub.4 .multidot.7H.sub.2 O which is produced from this process typically consists of crystals which are about 20-40 mesh in size and surface coated with about 0.5 -1.0% by weight H.sub.2 SO.sub.4 and about 2.0-4.0% by weight water. Although the resulting ferrous sulfate is currently characterized as a hazardous waste by the United States Environmental Protection Agency, much of it has been dumped indiscriminately regardless of adverse environmental consequences.
However, in recent years, much of the waste ferrous sulfate (e.g. FeSO.sub.4 .multidot.7H.sub.2 O) is being collected and used commercially in the production of Fe.sub.2 O.sub.3, and in various other applications.
It has also been determined that FeSO.sub.4 .multidot.H.sub.2 O (hereinafter "ferrous sulfate monohydrate") has considerable value as both an animal feed supplement (providing a substantial source of Fe) and as a micronutrient fertilizer for agricultural purposes. Ferrous sulfate monohydrate is conventionally obtained by heating either FeSO.sub.4 .multidot.7H.sub.2 O or FeSO.sub.4 .multidot.4H.sub.2 O at a temperature of above about 64 degrees C. Since the waters of crystallization in these materials are part of their molecular structures, the resulting ferrous sulfate monohydrate is reduced in size to a fine powder (e.g. not exceeding about -325 U.S. standard mesh or 44 microns) which is easily air-dispersed and has a low bulk-density. The ferrous sulfate in this form is suitable for use as an animal feed supplement, but will not function effectively as a plant micronutrient due to its small particle size. Instead, tests have shown that ferrous sulfate monohydrate is most effective for agricultural purposes when it is delivered in the form of a particle or pellet having a size of about -6+20 U.S. standard mesh. Pellets of this size enable a controlled distribution of nutrients into the soil, compared with powders (which disperse into the soil too rapidly) and larger pellets (which disperse too slowly).
Of the two uses for ferrous sulfate monohydrate listed above, agricultural applications are of primary importance. From a chemical perspective, ferrous sulfate monohydrate is an important source of soluble Fe. Accordingly, it will dissolve readily in water (e.g. rainwater), and is rapidly dispersed into the soil where it is thereafter drawn into plant root systems. Most soil materials west of the Mississippi river in the United States are soluble Fe-deficient, thereby creating a substantial need for Fe-based supplements such as ferrous sulfate monohydrate. In the northwestern parts of the United States, ferrous sulfate monohydrate may be used as a moss-control agent in view of its acidic pH. In citrus-growing regions of the United States, ferrous sulfate monohydrate may also be used to fertilize citrus trees which require substantial amounts of soluble Fe. For the purposes of this invention, the term "soluble Fe" shall be defined herein as amount of Fe by weight in a water-soluble Fe compound or composition. For example, ferrous sulfate monohydrate (a water-soluble Fe compound) has a theoretical Fe content of 32.9%, all of which is deliverable when used as a plant nutrient since the ferrous sulfate monohydrate is entirely water-soluble. In a ferrous sulfate monohydrate-containing particle/pellet, the % of soluble Fe therein would first involve 1) how much Fe (wt. %) is in the ferrous sulfate monohydrate (theoretically 32.9%) and 2) the amount of other ingredients in the particle/pellet which would correspondingly reduce the wt. % of Fe therein to a lower number. Normally, the amount of soluble Fe is readily determined by conventional chemical analytical techniques known in the art.
As noted above, it is desired that agricultural ferrous sulfate monohydrate have a particle/pellet size of about -6+20 U.S. standard mesh. In addition, it is also important that the particle/pellet have a spherical shape. The spherical shape and uniform size assist in maintaining a homogeneous mixture of materials in commercial packages of plant nutrients. This may not occur when non-uniform, nonspherical particles/pellets are used, thereby resulting in improper dispersion of nutrients during fertilizer application.
Furthermore, ferrous sulfate monohydrate pellets produced in accordance with the present invention should have no less than about 25% by weight soluble Fe, although an optimum soluble Fe concentration would be about 30% by weight or more. Soluble Fe concentrations less than about 25% may prevent efficient Fe delivery and utilization by plant systems.
To accomplish these goals, it is desired that ferrous sulfate monohydrate be produced in pellet form wherein each pellet has a size of about -6+20 U.S. standard mesh and is in prill (e.g spherical) form. The pellets must also be sufficiently hard to avoid crumbling during transport. Crumbling or breakage of the pellets will again prevent a homogeneous mixture of materials from being maintained.
The United States currently uses about 60,000-80,000 tons of ferrous sulfate monohydrate annually. Japan supplies about 70% of this amount. The rest is obtained domestically and from Korea and/or Mexico. However, the best available ferrous sulfate monohydrate from commercial sources is cubic in form rather than spherical, and of insufficient hardness. Cubic ferrous sulfate monohydrate particles are produced as a result of conventional production methods in which sheets of ferrous sulfate monohydrate are produced by drying FeSO.sub.4 .multidot.7H.sub.2 O to obtain ferrous sulfate monohydrate powder, and compressing the powder into loosely-bound sheets (e.g. by vacuum hot-pressing procedures known in the art) which are readily crushed and thereafter screened. This produces a product which does not blend well, and includes sharp pointed regions which are easily abraded, thereby producing an undesirable powder. Thus, a substantial need currently exists for a ferrous sulfate monohydrate pellet product which is spherical, has a particle size of about -6+20 U.S. standard mesh, has a soluble Fe level of not less than about 25% by weight, is readily manufactured, and is sufficiently durable to prevent breakage during transport. The present invention satisfies these needs in a new and unique manner, as described herein.